Details

To add API versioning to your .NET 8 Web API, follow these steps:

Install the Microsoft.AspNetCore.Mvc.Versioning NuGet package

In your Program.cs file, configure API versioning by adding the necessary services

// Configure API version
builder.Services.AddApiVersioning(options =>
{
   options.DefaultApiVersion = new ApiVersion(1, 0);
   options.AssumeDefaultVersionWhenUnspecified = true;
   options.ReportApiVersions = true;
   options.ApiVersionReader = ApiVersionReader.Combine(
       new UrlSegmentApiVersionReader(),
       new HeaderApiVersionReader("X-Api-Version"));
}).AddApiExplorer(options =>
{
   options.GroupNameFormat = "'v'V";
   options.SubstituteApiVersionInUrl = true;
});

Use the [ApiVersion] attribute on your controllers to specify which versions they support
For example, the version one controller could be…

[ApiController]
[ApiVersion("1.0")]
[Route("api/v{version:apiVersion}/[controller]")]
public class ProductsController : ControllerBase
{
   [HttpGet]
   public IActionResult Get()
   {
       return Ok("This is version 1.0");
   }
}

And the version two controller could be…

[ApiController]
[ApiVersion("2.0")]
[Route("api/v{version:apiVersion}/[controller]")]
public class ProductsV2Controller : ControllerBase
{
   [HttpGet]
   public IActionResult Get()
   {
       return Ok("This is version 2.0");
   }
}

And that's it!

You could improve the folder structure to something like this
to enhance maintainability

/Controllers
   /V1
       - ProductsController.cs
       - OrdersController.cs
       - CustomersController.cs
       - VendorsController.cs
   /V2
       - ProductsController.cs
       - OrdersController.cs
       - CustomersController.cs
       - VendorsController.cs
   /V3
       - ProductsController.cs
       - OrdersController.cs
       - CustomersController.cs
       - VendorsController.cs

To get Swagger support, add the following to your SwaggerGen

builder.Services.AddSwaggerGen(c =>
{
   c.SwaggerDoc("v1", new OpenApiInfo { Title = "Your API", Version = "v1" });
   c.SwaggerDoc("v2", new OpenApiInfo { Title = "Your API", Version = "v2" });
   c.SwaggerDoc("v3", new OpenApiInfo { Title = "Your API", Version = "v3" });
   // Configure other Swagger options if needed
});

Easy!